The Search for Other Earths

star field

2 Evolution of Planets

13.8 billion years ago, a sudden effusion of radiant energy emerged to form our universe. The actual instant of creation is shrouded from us by an interval we know nothing about. As the expansion of this energetic fireball continued under the influence of an unknown dark energy, the temperature dropped until unknown dark matter and ordinary matter (quarks, gluons, electrons, and other elementary particles) condensed out of the seething plasma.

 From one microsecond to one second after initiation of expansion, the temperature dropped enough for the mixture of quarks to condense into stable particles: protons and neutrons and mesons. The disappearance of free quarks released neutrinos. Having almost zero mass, these hot neutrinos left the plasma at close to the speed of light. They created a pervasive neutrino background throughout the universe, comparable to one created later by photons.

As the cooling continued, protons and neutrons combined into the nuclei of hydrogen, helium, and lithium, while, photons, the particles of light, remained trapped in interactions with free electrons. Three hundred thousand years later, the plasma suddenly became cool enough for the atomic nuclei to capture free electrons and form atoms. Hydrogen and small amounts of the other elements became an expanding sphere of hot neutral gas.

Light and Darkness

At this instant, photons were no longer trapped by electrons, and burst free in an incandescent sphere of light, rushing even faster than the neutrinos towards the boundary of time and space. We detect that light now as a microwave background gives us a picture of the universe when light broke free. We see a tiny pattern of quantum fluctuations (one part in 100,000) of density and temperature. The regions of slightly higher density are the ancient seeds of stars and galaxies.

Time passes. Space expands over vast distances, stretches the waves of light into redder and dimmer shades. Darkness infiltrates the universe; the temperature plunges downwards. For hundreds of millions of years, the universe became invisible, a void, an endless mist of newborn matter lost in the depths of time.

The remnants of minute quantum ripples in the density of matter stave off the death of the universe. Tenuous clouds of cold dark matter and ordinary matter move together under the influence of their own gravity. Matter congeals around miniscule ripples in space-time. Ripples thicken into threads. Threads flow into vast interconnected filaments that tie together the boundaries of the universe. Knots in the tangled filaments of matter thicken, twist and turn, sweeping up dark matter and the primordial elements into vast clouds of hydrogen. Gravity draws denser parts into dwarf galaxies—pregnant with the seeds of stars.


As turbulent eddies in denser regions of cold clouds matter gravitate inwards, they compress and grow hot. There is a struggle between the inward pull of gravity and the outward pressure of heat. As cold dark matter adds its gravity to that of normal gas, gravity wins the struggle. The first stars burst out of the dark ages. These are protostars, one-hundredth the size of our sun. They generate energy as their intense internal pressure and rising temperature fuses hydrogen into heavier elements. They live as long as the inward pull of gravity balances the outward pressure of their radiant heat.

These new stars devour hydrogen at enormous speed, and grow gigantically. The dark ages of the universe are driven back by a blaze of light. Groups of stars linked together by gravity begin to move in closed trajectories to form the earliest galaxies. Omnipresent dark matter enfolds these in a cloak of invisible mass, speeding up peripheral stars, stamping its presence on star-bright disks.

In a splendid effusion, the first stars burn a hundred times hotter than the sun for about a hundred million years. As a star exhausts its hydrogen, its temperature drops and its unconstrained gravitational energy pulls it inwards into a gigantic implosion followed by an explosion. This is a supernova that seeds the universe with heavy elements made by the fusion of hydrogen nuclei in the stellar furnace together with still heavier elements made during the explosive destruction of the star.

Supernova dust

Two views of a supernova remnant called SN 1987A — the left taken by the Herschel Space Observatory, and the right an enlarged view of the circled region at left, taken with the Hubble Space Telescope. The remnant is the remains of a supernova that occurred 170,000 light-years away, seen on Earth in 1987. The bright fuzzy spot in the Herschell image reveals the process generating dust in the universe. [More . . .] (Image Courtesy ESA/NASA-JPL/UCL/STScI)

This fresh matter cools and is swept up into clouds of gas, ice and dust from which new stars condense. The new heavy elements available to these stars enable them to burn much longer than hydrogen stars, by creating and consuming a series of new elements as fuels, until they reach iron, and implode again when unable to create heavier elements than iron. The main elements of life are assembled during the life of the star, except for a few heavier elements that originate in the resulting super nova explosion.

These second-generation stars burns for billions of years before they implode and spew out their newly formed elements into turbulent expanding clouds of gas, dust and ices. Within these clouds chemistry begins, as the new born elements interact at random or under the influence of radiation, gravity and magnetic fields to combine into chemical compounds such as water, carbon dioxide, ammonia, methane, and poly-aromatic hydrocarbons –the beginning steps on the path to life.


Time passes. After a billion years, the dwarf galaxies, each pulling on each, whirl, clash, and spiral together to form spinning wheels of hundreds of billions of stars. After seven billion years, one hundred billion galaxies populate our universe. But dark energy has grown as space itself grew. Increasingly it reduces the inward pull of gravity, driving the expansion of the universe still faster. The collapse of the galaxies back into themselves under the gravitational pull of filamentary chains of matter is defeated. The mass of the universe becomes mostly dark energy. The speed of expansion at the edge of the universe exceeds the speed of light, defining a cosmic horizon. Beyond it are regions of the universe we cannot see, of which we have no knowledge.

Eagle Nebula

The pillars at the center of the Eagle Nebula in this Kitt Peak image are regions of cold gas and dust where stars are forming. . Photo Credit: T.A.Rector  and B.A.Wolpa (NOAO/AURA/NSF)

But in local regions, gravity has the upper hand and continues to pull matter inward. Galaxies collide and merge. In the process, avalanches of stars push fountains of gas into interstellar space. Deaths of massive stars create black holes, dark cosmic monsters that draw radiation and matter—millions of stars—into their inner regions. The swirling vortex of stars and debris drawn towards these holes in space-time is crushed into whirling disk where enormous gravitational forces fuse atomic nuclei together, releasing energies that accelerate jets of matter through the galaxy and beyond. These jets and the explosive blasts of supernovae compress interstellar gas and create nurseries of new-born stars. But large amounts of hot gas escape a galaxy and form a vast sphere around it. As this cools it falls back into galactic rivers that refuel star birth. The black hole swallows stars; but as it grows. its jets create new stars. Ultimately, however, all the gas is used up and the birth of stars stops. The galaxy cools, turns red and dies.


In our present epoch, new stars continually condense from concentrations of gas, dust and rubble. As this occurs, nearby dusts and ice particles collapse into a disk that swirls around the new star. In the disk, over millions of years, gravitational effects and collisions oversee the birth of planets, moons, asteroids, and comets.

The planets that circle a star are born in violence and fury as gases, dusts and ices collide and fuse into liquids, ice or rocks. In turn, these small bodies crash violently together to create planetismals, mini planets. The immense heat of their formation melts rocks and metals. Molten iron may sink and form an iron core under a surface of siliceous rocks, with the whole planetismal spinning into a sphere. Other planetismals may form with icy or hydrogen cores. Planets are built as planetismals collide and fuse together. Some are rocky, some are formed from ices. Depending on their distance from their star, the icy planets may become water worlds or gas giants. Further out are they continue as ice planets. Material not grasped by planets forms moons, asteroids, and comets.

Gravity sorts planets into different orbits. In our Solar System, we find planets drawn closest to our Sun by gravity are largely made of iron and rock. Further out, giant gas planets have formed; beyond these, a giant belt of icy worlds like Pluto has formed. But we find a different pattern in planetary systems around other stars. The actual composition of a planet varies with the mixture of planetismals that make it up. Later, the planets around a star may indulge in a shuffle towards or away from their star as their own gravitational fields interact. Many planets become incandescent cinders as the star's gravity brings them close to its surface. Violent collisions may break up others or hurl them from the star to wanderer alone in interstellar space. There are billions such wanderers in our own galaxy.

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